Present day telecommunications carriers, such AT&T, carry large volumes of telecommunications traffic across their networks. While most carriers strive for high reliability, disruptions in their networks can and do occur. Such disruptions are often attributable to a failure in a link between two network nodes, each node typically comprising a telephone central office or network control center. The links, which may each take the form of a copper cable, optical fiber or a radio channel, may fail because of an act of nature, such as a flood, ice storm, hurricane or other weather-related occurrence. Link failures are sometimes man-made. Not infrequently, a contractor may accidentally sever a cable or fiber link during excavation along the link right-of way.
Regardless of the manner in which a communication link is severed, a link failure invariably disrupts communications services. For example, the loss of a single fiber in an optical cable usually results in many blocked calls. Each blocked call represents a loss of revenue for the carrier carrying that call. Thus, rapid restoration of traffic is critical. Typically, telecommunications carriers achieve traffic restoration by routing traffic on alternate routes. Since spare capacity often exists on many routes, traffic restoration is a matter of determining where such spare capacity exists and then establishing a path utilizing such spare capacity to bypass the severed link.
U.S. Pat. No. 5,182,744, "Telecommunications Network Restoration Architecture," issued on Jan. 26, 1993, in the name of James Askew et al. and assigned to AT&T (incorporated by reference herein) discloses a restoration technique for routing telecommunications traffic on an alternate route in the event of a severed communications link. The Askew et al. technique utilizes communications monitors at the nodes to detect the disruption of traffic across the communications links. Should a disruption occur because of a failed link, the monitor at one or both of the affected nodes notifies a central facility that determines an alternate route over which the traffic can bypass the failed link. After finding a restoration route, the central facility directs the nodes to conduct a continuity test of the restoration route. Upon successful completion of the continuity test, the disrupted traffic passes over the restoration route.
While the Askew et al. restoration technique represents a significant advance over past approaches, the continuity test performed prior to restoration is not instantaneous. At present, restoration of one hundred DS3 signals takes about five minutes. Even though such an interval may seem insignificant, as many as 50,000 calls may be blocked during this time. Thus, there is a need for a technique achieves network restoration quickly, to reduce the incidence of blocked calls.